Warning: just reading this article might make your skin crawl. Thinking about itching, seeing people scratch, looking at pictures of bedbugs or other itch inducers—all can bring on an irresistible urge to flick away that irksome feeling.

But itching—“pruritus,” to physicians—is more than an occasional nuisance. The sensation, which arises from an irritation of the nerve cells along the skin, serves as a helpful warning about potential hazards such as insects or foreign materials—and scratching is often a simple and effective method for dealing with them. Itching is also the main symptom of many skin diseases and appears in some systemic conditions, such as chronic renal disease, cirrhosis and some types of cancer.